The addition of foaming agents to firefighting water streams is known and is particularly useful for fighting fires in industrial factories, chemical plants, petrochemical plants and petroleum refineries. The use of firefighting foam requires that a foam concentrate be mixed and added at a constant proportion to the water stream. When the foam extinguisher solution is delivered, it generates a foam which effectively extinguishes the flames of chemical and petroleum fires as well as Class A materials which would otherwise not be effectively extinguished by the application of water alone.
It is important that the foam concentrate be added to the water stream in the appropriate proportion for the instant application. When an excessive amount of foam concentrate is added, a lower fire-extinguishing quality can result due to an increased foam viscosity which limits the flowability of the foam and the ability of the foam to be spread on the fire. Further, the addition of excessive amounts of concentrate to the water stream increases the cost of the use of the foam and the frequency at which the foam concentrate supply must be replenished at the scene. Replenishing the foam concentrate is cumbersome because the concentrate is typically contained in a pressurized vessel or reservoir. Still more important, with currently-available foam concentrates, the foam:water ratio can be less than 1:99 and may need to be controlled in precise increments of 0.2% or less. As a result, accuracy in the foam proportioning system is important.
Generally, there are two currently available types of foam proportioning systems. One such type involves the drawing of the foam-forming concentrate into the water stream by an in-line or by-pass foam eductor. Two problems are associated with such eductor devices. First, the foam to water ratio is at times fixed and/or often not accurate. Second, eductor devices create a substantial pressure drop across the eductor which limits the flow through the system and the maximum spray distance. As a result, firefighters using a separate eductor system to apply foam need to stand close to the fire, as less usable pressure is available.
The second type of system is commonly referred to as a balanced pressure proportioning system. Balanced pressure proportioning systems supply the foam concentrate to the water stream under pressure and therefore often include a pump and a control valve. As a result, the pressure drop across the eductor is reduced. However, balanced pressure proportioning systems are rather cumbersome because they require a separate foam pump and a power source for the foam pump. As a result, balanced pressure proportioning systems are also more typically expensive than the separate eductor systems.
Some balanced pressure proportioning systems do not require a separate foam pump but, instead, store the foam concentrate in a pressurized bladder tank which includes one compartment containing the foam concentrate and a second compartment for holding water. The two compartments are separated by a bladder. Or with batch mixing the water and foam concentrate can be co-mingled. The tank is connected to the water intake line and is operated under the line pressure. However, because the foam concentrate is contained within a pressurized tank, the replenishing of the foam concentrate in the tank requires that the tank be manually refilled, or depressurized and hand filled, and therefore causes a significant time loss in the event the foam concentrate needs to be replenished.
Still another disadvantage of currently-available balanced pressure proportioning systems is the difficulty in controlling these systems. Specifically, currently-available systems include a metering valve to control the rate at which foam is injected into the water stream. However, the control of the metering valve either requires the use of an electrical, mechanical or hydraulic connection between the metering valve and a control panel. As a result, a line, albeit electrical, mechanical or hydraulic, is required to connect the metering valve to the control panel. The use of this additional line, or any additional line for that matter, is cumbersome in a firefighting environment.